The risk of HIV transmission during oral sex with a man (i.e. sucking his penis) is fairly low, but transmission may sometimes happen. Taking his cum in your mouth increases the risk.
The virus may get into your bloodstream directly from your mouth, if there are cuts, ulcers, sores, abrasions, or gum disease. So you don't need to "eat" or swallow the cum for this risk to exist. In fact once the semen gets into the stomach, the acids in the stomach can kill the virus.
It's also possible to pick up other sexually transmitted diseases during oral sex. Nonetheless the risks of picking up an infection during oral sex are far lower than during anal or vaginal sex when a condom isn't used.
More on Oral Sex at TheBody.com
To find out more about the risk from oral sex, we recommend the following articles:
In addition, our Q&A experts sometimes address questions about oral sex in our "Ask the Experts" forums. Here are some of those questions and our experts' responses:
- Safe ways of consuming semen
I would like to introduce semen into my diet. I want to be certain that it can be done in a safe way (in case the semen in HIV infectious). Would adding semen into a scrambled egg mixture and then cooking it in a frying pan kill any HIV present?
- Forced to Swallow
He came in my mouth and I couldn't pull away because he was holding the back of my head. I swallowed half of it before I spat out the rest. He said if I was worried about HIV I should have swallowed all of it because it's safer.